1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a planar mirror finish, and more particularly to a polishing apparatus for polishing a workpiece by pressing a polishing pad or a grinding plate and the workpiece against each other while moving them in sliding contact with each other.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithography process can form narrower interconnections, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surface of semiconductor wafers has been to polish semiconductor wafers by polishing apparatus.
Heretofore, polishing apparatus for polishing semiconductor wafers comprises a turntable with a polishing pad attached thereto and a top ring for holding a semiconductor wafer to be polished. The top ring which holds a semiconductor wafer to be polished presses the semiconductor wafer against the polishing pad on the turntable. While an abrasive liquid is being supplied to the polishing pad, the top ring and the turntable are rotated about their own axes to polish the surface of the semiconductor wafer to a planar mirror finish.
FIG. 1 of the accompanying drawings shows a conventional polishing apparatus. As shown in FIG. 1, the conventional polishing apparatus comprises a turntable 5 with a polishing pad 6 attached to an upper surface thereof, a top ring 1 for holding a semiconductor wafer 4 which is a workpiece to be polished while rotating and pressing the semiconductor wafer 4 against the polishing pad 6, and an abrasive liquid supply nozzle 9 for supplying an abrasive liquid Q to the polishing pad 6. The upper surface of the polishing pad 6 provides a polishing surface. The top ring 1 is connected to a top ring drive shaft 8, and supports on its lower surface a resilient mat 2 such as of polyurethane or the like. The semiconductor wafer 4 is held on the top ring 1 in contact with the resilient mat 2. The top ring 1 also has a cylindrical guide ring 3 mounted on a lower outer circumferential surface thereof for preventing the semiconductor wafer 4 from being disengaged from the lower surface of the top ring 1 while the semiconductor wafer 4 is being polished. The guide ring 3 is fixed to the top ring 1 against relative movement in the circumferential direction. The guide ring 3 has a lower end projecting downwardly beyond the lower supporting surface of the top ring 1. The guide ring 3 holds the semiconductor wafer 4 on the lower supporting surface of the top ring 1 against dislodgment from the top ring 1 due to frictional forces developed between the semiconductor wafer 4 and the polishing pad 6 while the semiconductor wafer 4 is being polished.
In operation, the semiconductor wafer 4 is held against the lower surface of the resilient mat 2 on the top ring 1, and pressed against the polishing pad 6 by the top ring 1. The turntable 5 and the top ring 1 are rotated about their own axes to move the polishing pad 6 and the semiconductor wafer 4 relatively to each other in sliding contact for thereby polishing the semiconductor wafer 4. At this time, the abrasive liquid Q is supplied from the abrasive liquid supply nozzle 9 to the polishing pad 6. The abrasive liquid Q comprises, for example, an alkaline solution with fine abrasive grain particles suspended therein. Therefore, the semiconductor wafer 4 is polished by both a chemical action of the alkaline solution and a mechanical action of the fine abrasive grain particles. Such a polishing process is referred to as a chemical and mechanical polishing (CMP) process.
Another known polishing apparatus employs a grinding plate made of abrasive grain particles bonded by a synthetic resin for polishing a workpiece. The grinding plate is mounted on the turntable, and an upper surface of the grinding plate provides a polishing surface. Since this polishing apparatus does not employ a soft polishing pad and a slurry-like abrasive liquid, it can polish the workpiece to a highly accurate finish. The polishing process by the grinding plate is also advantageous in that it is less harmful to the environment because it discharges no waste abrasive liquid.
The conventional polishing apparatus shown in FIG. 1 has a spherical bearing 7 positioned between the top ring 1 and the top ring drive shaft 8. The spherical bearing 7 allows the top ring 1 to be tilted quickly with respect to the top ring drive shaft 8 even when the top ring 1 encounters a small slant on the upper surface of the turntable 5. The top ring drive shaft 8 is kept in driving engagement with the top ring 1 by a torque transmission pin 107 on the top ring drive shaft 8 and torque transmission pins 108 on the top ring 1. The torque transmission pins 107, 108 are held in sliding point-to-point contact with each other. When the top ring 1 is tilted with respect to the top ring drive shaft 8, the torque of the top ring drive shaft 8 is smoothly and reliably transmitted to the top ring 1 because the torque transmission pins 107, 108 change their point of contact while transmitting the torque.
The above conventional polishing apparatus are problematic in that while polishing a workpiece, the polishing apparatus suffer large vibrations owing to frictional forces developed between the turntable and the top ring with the workpiece interposed therebetween. An analysis suggests that such large vibrations are caused by a combined action of resistant forces by the rotating top ring and the rotating turntable which are rotated independently of each other, such resistant forces being dependent on frictional forces developed between the surface of the workpiece and the surface of the polishing pad or grinding plate and restoring forces exerted by the top ring drive shaft and a turntable drive shaft.
When the vibrations become large the polished surface of the workpiece develops polish irregularities or scratches or other surface damage, and hence the workpiece cannot be polished stably. The vibrations may become so intensive that the workpiece may be forcibly detached from the top ring and no will be polished.
It is therefore an object of the present invention to provide a polishing apparatus which is capable of preventing undue vibrations during a polishing process and of stably polishing a workpiece to a planar mirror finish.
According to the present invention, there is provided a polishing apparatus for polishing a workpiece to a planar mirror finish by pressing the workpiece against a polishing surface while keeping the workpiece and the polishing surface in sliding motion, comprising a holding member for holding the workpiece, a mechanism for rotating the holding member, and a bearing supporting an outer circumferential surface of the holding member, for suppressing vibrations transmitted to the holder while the workpiece is being polished.
The outer circumferential surface of the holding member which holds the workpiece to be polished is rotatably supported by the bearing for suppressing vibrations of the holding member. Vibrations produced owing to a combined action of frictional forces developed on the surface being polished and restoring forces exerted by drive shafts of the holding member and the abrasive member, are maximized on the holding member supported by the drive shaft which is relatively small in diameter. Therefore, since the holding member is rotatably supported at its outer circumferential surface by the bearing, vibrations of the holding member are suppressed, and hence vibrations of the polishing apparatus in its entirety are also suppressed. Consequently, even when the rotational speeds of the workpiece and the abrasive member increase or the pressure applied therebetween increases to develop a build up of frictional force the polishing apparatus is effectively prevented from being unduly vibrated, and can polish the workpiece stably under desired operating conditions.
As described above, vibrations of the holding member can be suppressed because the holding member is rotated with its outer circumferential surface being rotatably supported by the bearing. This structure is also applicable to other polishing apparatus than polishing apparatus which have a top ring and a turntable. Specifically, the structural details are applicable to a cup-type polishing apparatus in which the workpiece is arranged with its surface to be polished facing upwardly and the abrasive member rotates and presses against the workpiece, and also to a scrolling-type polishing apparatus in which the grinding plate or polishing pad is arranged with its polishing surface facing upwardly and the workpiece is arranged with its surface to be polished facing downwardly against the grinding plate or polishing pad, which is caused to make a scrolling motion such as circulate orbital motion to polish the workpiece.
The bearing may comprise a mechanical bearing or a non-contact-type bearing such as a magnetic bearing.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.